1. Field of the Invention
The present invention relates to a large-aperture medium-range telephoto lens system and, more particularly, to a focusing method of a fast medium-range telephoto lens system.
2. Related Background Art
In recent years, along with the widespread use of auto-focusing single-lens reflex cameras, various focusing methods each of which attains focusing by moving only some lens components in a lens system have been proposed. Of these methods, as focusing methods for a so-called telephoto lens system, the following methods are known. In a telephoto lens system which comprises a first lens unit of a positive refractive power, a second lens unit of a negative refractive power, and a third lens unit of a positive refractive power, a so-called internal focusing method which attains focusing by only moving the second lens unit of the negative refractive power is known. On the other hand, in a telephoto lens system which comprises a front lens unit of a positive refractive power and a rear lens unit of a positive refractive power, a rear focusing method which attains focusing by moving only the rear lens unit is known.
However, in the internal focusing method, asymmetry of the optical system is strong, and the field of curvature and astigmatism vary considerably upon focusing. For this reason, it is difficult to apply this method to a lens system with a large field angle.
On the other hand, in the above-mentioned rear focusing method, a variation in image surface can be suppressed by almost symmetrically arranging the front and rear lens units, but then spherical aberration varies greatly.